1. Field of the Invention
The present invention relates to a laser diode module (LD module) for optically coupling a laser diode (LD) and an optical fiber.
2. Description of the Related Art
In an optical communication system using an optical fiber as a transmission line, an LD module is used to introduce light emitted from a light emitting component (e.g., laser diode) into the optical fiber. In the LD module, the light emitting component and the incident end face of the optical fiber are fixed in a given positional relationship, and a condenser lens is interposed between the light emitting component and the incident end face of the optical fiber. In this kind of LD module, the positional relation between components has a direct effect on optical coupling efficiency, so that the components must be positioned with an extremely high accuracy as less than or equal to 1 xcexcm. Further, this positioning accuracy must be maintained for a long period of time.
The components mounted on a printed wiring board built in a communications device are generally classified into a surface mount type and a through hole mount type. A typical example of the surface mount type components is an LSI, which has a form called a flat package. Such a component is soldered to the printed wiring board by a reflow soldering process. This process is performed by printing a solder paste on the printed wiring board, making the surface mount type component stick to the printed solder paste, and heating the whole in a conveyor oven to a solder surface temperature of 220xc2x0 C. or higher.
A typical example of the through hole mount type components is a large-capacity capacitor or a multi-terminal (200 or more terminals) LSI. The multi-terminal LSI has a terminals form called a PGA (Pin Grid Array). Such a through hole mount type component is soldered to the printed wiring board by a flow soldering process. This process is performed by inserting the terminals of the through hole mount type component into through holes of the printed wiring board, and putting the printed wiring board into a solder bath heated at about 260xc2x0 C. from the side opposite to its component mounting surface.
In mounting an optical module such as an LD module on the printed wiring board by soldering like the surface mount type component or the through hole mount type component, a so-called pigtail type of optical module with an optical fiber cord is not suitable as the optical module. That is, the optical fiber cord usually has a nylon coating, and the nylon coating has a low resistant to heat at about 80xc2x0 C., so that it is melted in the soldering step. Furthermore, the optical fiber cord itself invites inconveniences in accommodation and handling at a manufacturing location, causing a remarkable reduction in mounting efficiency to the printed wiring board. Accordingly, to allow a soldering process for the optical module and reduce a manufacturing cost, the application of a so-called receptacle type of optical module is indispensable, excluding the optical fiber cord.
In general, a conventional receptacle LD module has a female type receptacle structure adapted to receive a ferrule of an optical connector as disclosed in Japanese Patent Laid-open No. Hei 11-295559. The female type LD module is composed of an LD package, a receptacle assembly for receiving the ferrule of the optical connector, and a sleeve interposed between the LD package and the receptacle assembly for allowing triaxial adjustment of the receptacle assembly relative to a laser diode (LD) mounted in the LD package.
The conventional female type receptacle LD module is assembled by the following method. First, the LD package, the sleeve, and the receptacle assembly in which a lens is inserted and fixed are set on an assembling jig. An LD driving power supply is connected to the LD in the LD package, and a laser beam is emitted from the LD. On the other hand, the ferrule projecting from one end of the optical connector is inserted into the receptacle assembly, and an optical power meter for monitoring the light quantity of the laser beam entered the optical fiber in the ferrule is connected to the other end of the optical connector.
In this condition, the lens is moved relative to the laser beam from the LD along an optical axis (in a Z-axis direction) and in the directions perpendicular to the optical axis (in X-axis and Y-axis directions) to search for a position where the reading on the optical power meter shows a maximum value. When the reading on the optical power meter shows a maximum value, the adjustment of the lens is finished. Thereafter, the receptacle assembly and the sleeve are welded together at the interface therebetween, and the sleeve and the LD package are also welded together at the interface therebetween.
A semiconductor laser module in the above-mentioned receptacle LD module (Japanese Patent Laid-open No. Hei 11-295559) is assembled by fixing a receptacle in which a lens is inserted and fixed, a semiconductor laser apparatus retained to a laser holder, and a ring on an assembling jig, next inserting into the receptacle one end of a ferrule in which an optical fiber is inserted and fixed, the optical power meter being connected to the other end of the ferrule, next adjusting the optical axis, and finally fixing the laser holder and the ring at the interface therebetween and the ring and the receptacle at the interface therebetween by welding, bonding, etc.
Thus, the adjustment of the optical axis is carried out after inserting into the receptacle one end of the ferrule in which the optical fiber is inserted and fixed. However, the ferrule and the receptacle are not connected and the preset positional relation therebetween is not always maintained with a high accuracy even after the measurement. Further, also in the case of engaging the ferrule of the connector with a projecting portion of the receptacle, the insertion length of the ferrule slightly changes every time of insertion of the ferrule, so that the positional relation between the ferrule and the receptacle cannot be accurately maintained.
Further, there is a case that the incident end face of the ferrule is obliquely cut to prevent the reflection of a laser beam emitted from a semiconductor laser chip. In this case, the relation between the direction of cutting of the ferrule and the position of the semiconductor laser chip must be maintained constant. If rotation of the ferrule about its axis occurs during insertion of the ferrule into the receptacle, the ideal positional relation is lost to cause a reduction in reflection prevention efficiency. Accordingly, such a reduction in reflection prevention efficiency due to the rotation of the ferrule cannot be suppressed only by the idea of making the insertion length of the ferrule in the receptacle constant.
It is therefore an object of the present invention to provide a laser diode module which can be easily assembled and adjusted in optical axis and is suitable for maintenance of the positional relation between the components after adjustment of the optical axis.
It is another object of the present invention to provide an assembling method for a laser diode module which can be easily adjusted in optical axis.
In accordance with an aspect of the present invention, there is provided a laser diode module comprising a laser diode assembly including a base, a carrier fixed to the base, a laser diode mounted on the carrier, a cap fixed to the base so as to surround the laser diode, and a holder fixed to the base so as to surround the cap; a lens-fiber assembly including a casing having a first end, a second end, a first hole having a first diameter and a first axis, and a second hole having a second diameter smaller than the first diameter and a second axis offset from the first axis, the second hole communicating with the first hole, a lens inserted and fixed in the first hole from the first end of the casing, and a ferrule with an optical fiber embedded therein, the ferrule having a slant polished first end and a second end, the ferrule being inserted and fixed in the second hole from the second end of the casing so that a given distance is defined between the first end of the ferrule and the lens and that the second end of the ferrule projects from the second end of the casing; and a sleeve having a first end fixed to the holder and a second end to which the first end of the casing is fixedly inserted; the first end of the ferrule being positioned so that a portion of the first end of the ferrule radially farthest from the first axis of the first hole becomes axially farthest from the lens.
Preferably, the slant angle of the slant polished first end of the ferrule is set in the range of about 4xc2x0 to about 8xc2x0 with respect to a plane perpendicular to the axis of the ferrule. Further, the casing further has a third hole for making communication of the first and second holes between the lens and the ferrule with the ambient air, and a pin for closing the third hole.
In accordance with another aspect of the present invention, there is provided an assembling method for a laser diode module using a lens-fiber assembly including a casing having a first end, a second end, and a through hole, a lens inserted and fixed in the through hole, and a ferrule with an optical fiber embedded therein, the ferrule having a slant polished first end and a second end, the ferrule being inserted and fixed in the through hole so that a given distance is defined between the lens and the first end of the ferrule, the assembling method comprising the steps of setting a laser diode assembly having a laser diode and a holder, a sleeve, and the lens-fiber assembly on an assembling jig; optically connecting an optical power meter to the second end of the ferrule; bringing a first end of the sleeve into contact with the holder, and inserting the first end of the casing into the sleeve from a second end thereof; moving the lens-fiber assembly relative to the laser diode along an optical axis and in the directions perpendicular to the optical axis while monitoring the power of a laser beam emitted from the laser diode by using the optical power meter; and welding the sleeve and the holder and welding the sleeve and the casing at a position where the reading on the optical power meter shows a maximum value.
In accordance with a further aspect of the present invention, there is provided a laser diode module comprising a laser diode assembly including a base, a carrier fixed to the base, a laser diode mounted on the carrier, a cap fixed to the base so as to surround the laser diode, and a holder fixed to the base so as to surround the cap; and a lens-fiber assembly including a casing having a first end, a second end, a first hole having a first diameter and a first axis, and a second hole having a second diameter smaller than the first diameter and a second axis offset from the first axis, the second hole communicating with the first hole, a lens inserted and fixed in the first hole from the first end of the casing, and a ferrule with an optical fiber embedded therein, the ferrule having a slant polished first end and a second end, the ferrule being inserted and fixed in the second hole from the second end of the casing so that a given distance is defined between the first end of the ferrule and the lens and that the second end of the ferrule projects from the second end of the casing; the first end of the casing being fixed to the holder; the first end of the ferrule being positioned so that a portion of the first end of the ferrule radially farthest from the first axis of the first hole becomes axially farthest from the lens.
In accordance with a still further aspect of the present invention, there is provided an assembling method for a lens-fiber assembly, comprising the steps of preparing a casing having a first end, a second end, a first hole having a first diameter and a first axis, and a second hole having a second diameter smaller than the first diameter and a second axis offset from the first axis, the second hole communicating with the first hole; inserting a lens from the first end of the casing into the first hole of the casing, and fixing the lens at a given position; inserting a ferrule having a slant polished first end, a second end, and an optical fiber embedded therein from the second end of the casing into the second hole of the casing so as to satisfy a positional relation that a given distance is defined between the first end of the ferrule and the lens and that a portion of the first end of the ferrule radially farthest from the first axis of the first hole becomes axially farthest from the lens; and fixing the ferrule.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.